6-Monoacetylmorphine (6-MAM), being a unique metabolite of heroin, is routinely tested in urine samples to monitor heroin use. However, detection of 6-MAM-related opiates such as morphine is known to be affected by in vitro urine adulteration using oxidizing adulterants such as potassium nitrite. This study aimed to investigate the fate of 6-MAM after exposure to nitrite and to identify any formed oxidation products that may potentially be used for monitoring heroin abuse despite nitrite adulteration. Potassium nitrite (0.05 M and 0.6 M) was reacted with 6-MAM (5–10,000 ng/mL) in both water and blank urine with pH adjusted to range from 3 to 8. Following reaction at room temperature for varying periods, the reaction mixtures were monitored by both the CEDIA® Heroin Metabolite (6-AM) immunoassay and liquid chromatography-mass spectrometry (LC-MS) methods. Structural elucidation of the isolated oxidation products was based on mass spectrometry and nuclear magnetic resonance spectroscopic evidence. Nitrite, under acidic environment (pH < 7), was shown to be effective in masking the detection of 6-MAM by both the CEDIA® immunoassay and the LC-MS methods. 2-Nitro-6-monoacetylmorphine (2-nitro-MAM) was identified as the sole oxidation product, which remained detectable in urine for at least 11 days under the experimental conditions investigated. 2-Nitro-MAM was detectable in a urine sample of a heroin user after nitrite exposure. 2-Nitro-MAM has shown potential to serve as a marker for monitoring heroin abuse when urine is adulterated with nitrite. Certification of 2-nitro-MAM reference standard for further development of its quantitative testing methods is thus warranted.